"Criminals send an email in which they give a personal password belonging to the target, and which was apparently obtained years ago by mass internet piracy, or they may claim that the user's computer has been pirated," police said in a statement.

"They claim to be in possession of personal intimate pictures or videos obtained through the computer's webcam when the user was visiting pornographic sites and they demand payment in bitcoin or other virtual currencies within 24 hours to stop them from publishing them," it said.

Demands are typically for between $400 and $2,900, and the cyber criminals threaten to send the videos to the person's "loved ones" unless they pay up.

Police recommend that targets of the shakedown stay calm and refuse to pay because the blackmailers are usually bluffing and have no video because no computers were actually hacked.

Police said they created a specific email address for these types of extortion attempts, and were receiving around 100 messages per day flagging possible cases.